The smokelet particle type is useful for creating smoke trails, puffy clouds and other organic looking shadowed effects.
Step-by-step instructions on using the Smokelet particle
Start with a new comp (Ctrl/Cmd-N), make it 640*480 at 30 fps. 5 seconds long.
In order to explain the smokelet particle we'll start with one of the presets that uses it - select the "t_Smoke" preset. Go to time 2:00 (2 seconds):
All the special settings for the Smokelet are in Options. Let's try changing the shadow color to a green shade. In the "Smokelet Shadow" section of Options, set R=0, G=64, B=0. Then click OK:
The setting called "Color Strength" controls how strong the RGB (from above) is to be weighted in against the particle color when computing the shadow. The setting called "Opacity" controls how strong the shadow is.
Controlling the positioning of the shadow
By default, the smokelet shadow is placed as if the light source was up on the left hand side and slightly behind. However, the smokelet shadow position can be controlled specifically if a light named "Shadow" is present in the composition (this name can be changed in Options).
Create a point light named "Shadow". (Layer>New>Light). The shadowing is most apparent if the light source is slightly behind the lit object so set the light Z position to 100. Then grab the light and move it in the XY plane and see how that affects the smoke. A parallel light may also be used as shadow light.